Transversely excited film bulk acoustic resonator using rotated y-x cut lithium niobate

ABSTRACT

Acoustic resonator devices, filters, and methods are disclosed. An acoustic resonator includes a substrate and a lithium niobate (LN) plate having front and back surfaces. The back surface is attached to a surface of the substrate except for a portion of the LN plate forming a diaphragm that spans a cavity in the substrate. An interdigital transducer (IDT) is formed on the front surface of the LN plate such that interleaved fingers of the IDT are disposed on the diaphragm. The LN plate and the IDT are configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic wave in the diaphragm. The Euler angles of the LN plate are [0°, β, 0° ], where β is greater than or equal to 0° and less than or equal to 60°.

RELATED APPLICATION INFORMATION

This patent claims priority to provisional application 62/904,133,titled WIDE BAND BAW RESONATORS ON 120-130 Y-X LITHIUM NIOBATESUBSTRATES, filed Sep. 23, 2019. This patent is a continuation-in-partof application Ser. No. 16/689,707, titled BANDPASS FILTER WITHFREQUENCY SEPARATION BETWEEN SHUNT AND SERIES RESONATORS SET BYDIELECTRIC LAYER THICKNESS, filed Nov. 20, 2019, which is a continuationof application Ser. No. 16/230,443, filed Dec. 21, 2018, titledTRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR, now U.S. Pat. No.10,491,192, which claims priority from the following provisionalapplications: application 62/685,825, filed Jun. 15, 2018, entitledSHEAR-MODE FBAR (XBAR); application 62/701,363, filed Jul. 20, 2018,entitled SHEAR-MODE FBAR (XBAR); application 62/741,702, filed Oct. 5,2018, entitled 5 GHZ LATERALLY-EXCITED BULK WAVE RESONATOR (XBAR);application 62/748,883, filed Oct. 22, 2018, entitled SHEAR-MODE FILMBULK ACOUSTIC RESONATOR; and application 62/753,815, filed Oct. 31,2018, entitled LITHIUM TANTALATE SHEAR-MODE FILM BULK ACOUSTICRESONATOR. This patent is a continuation-in-part of application Ser. No.16/381,141, filed Jun. 11, 2019, titled SOLIDLY MOUNTEDTRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR, which is acontinuation-in-part of application Ser. No. 16/230,443, and claimspriority from provisional patent application 62/753,809, filed Oct. 31,2018, titled SOLIDLY MOUNTED SHEAR-MODE FILM BULK ACOUSTIC RESONATOR,and provisional patent application 62/818,564, filed Mar. 14, 2019,titled SOLIDLY MOUNTED XBAR. All of these applications are incorporatedherein by reference.

This application is related to application Ser. No. 16/518,594, filedJul. 22, 2019, entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTICRESONATOR USING ROTATED Z-CUT LITHIUM NIOBATE.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to filters, oscillators, sensors and other radiofrequency devices using acoustic wave resonators, and specifically tofilters for use in communications equipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a pass-band or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is better than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. The currentLTE™ (Long Term Evolution) specification defines frequency bands from3.3 GHz to 5.9 GHz. Some of these bands are not presently used. Futureproposals for wireless communications include millimeter wavecommunication bands with frequencies up to 28 GHz.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave (BAW) resonators, film bulkacoustic wave resonators (FBAR), and other types of acoustic resonators.However, these existing technologies are not well-suited for use at thehigher frequencies proposed for future communications networks.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view and two schematic cross-sectionalviews of a transversely excited film bulk acoustic resonator (XBAR).

FIG. 2 is an expanded schematic cross-sectional view of a portion of theXBAR of FIG. 1.

FIG. 3 is an alternative expanded schematic cross-sectional view of anXBAR.

FIG. 4 is a graphic illustrating a shear horizontal acoustic mode in anXBAR.

FIG. 5 is a graphical representation of Euler angles.

FIG. 6 is a chart of the e24 piezoelectric coefficient of a lithiumniobate plate with Euler angles [0°, β, 0° ] as a function of R.

FIG. 7 is a chart of the e14 and e15 piezoelectric coefficients of alithium niobate plate with Euler angles [0°, β, 0° ] as functions of R.

FIG. 8 is a chart comparing the admittances of XBARs formed on rotatedY-X lithium niobate and Z cut lithium niobate.

FIG. 9 is a chart of the resonance and anti-resonance frequencies of anXBAR on rotated Y-X lithium niobate as functions of Z-axis tilt angle R.

FIG. 10 is a simplified schematic circuit diagram of a bandpass filterusing XBARs.

FIG. 11 is a chart of the transmission through, and the returnreflection from, an embodiment of the bandpass filter of FIG. 9.

FIG. 12 is a flow chart of a process for fabricating an XBAR.

FIG. 13 is a flow chart of a process for fabricating a solidly mountedXBAR.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

FIG. 1 shows a simplified schematic top view and orthogonalcross-sectional views of a transversely excited film bulk acousticresonator (XBAR) 100. XBARs were first described in application Ser. No.16/230,443. XBAR resonators such as the resonator 100 may be used in avariety of RF filters including band-reject filters, band-pass filters,duplexers, and multiplexers. XBARs are particularly suited for use infilters for communications bands with frequencies above 3 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on asurface of a piezoelectric plate 110 having essentially parallel frontand back surfaces 112, 114, respectively. The piezoelectric plate is athin single-crystal layer of lithium niobate. The piezoelectric plate iscut such that the orientation of the X, Y, and Z crystalline axes withrespect to the front and back surfaces is known and consistent. Inparticular, the piezoelectric plate 110 is rotated Z-cut, which is tosay the Z crystalline axis is inclined by a small rotation anglerelative to the normal to the front and back surfaces 112, 114. As willbe discussed in further detail, the small rotation angle is defined bythe second Euler angle of the piezoelectric plate.

The back surface 114 of the piezoelectric plate 110 is attached to asurface of the substrate 120 except for a portion of the piezoelectricplate 110 that forms a diaphragm 115 spanning a cavity 140 formed in thesubstrate. The portion of the piezoelectric plate that spans the cavityis referred to herein as the “diaphragm” 115 due to its physicalresemblance to the diaphragm of a microphone. As shown in FIG. 1, thecavity 140 is a hole though the substrate 110. In other configurations,the cavity 140 may be a recess in the substrate 120. Also as shown inFIG. 1, the diaphragm 115 is contiguous with the rest of thepiezoelectric plate 110 around all of a perimeter 145 of the cavity 140.In this context, “contiguous” means “continuously connected without anyintervening item”. In other configurations, there may be openingsthrough the piezoelectric plate 110 (for example to allow etching of thecavity beneath the piezoelectric plate). In this case, the diaphragm 115will be contiguous with the rest of the piezoelectric plate 110 aroundat least 50% of the perimeter 145 of the cavity.

The substrate 120 provides mechanical support to the piezoelectric plate110. The substrate 120 may be, for example, silicon, sapphire, quartz,or some other material or combination of materials. The back surface 114of the piezoelectric plate 110 may be bonded to the substrate 120 usinga wafer bonding process. Alternatively, the piezoelectric plate 110 maybe grown on the substrate 120 or attached to the substrate in some othermanner. The piezoelectric plate 110 may be attached directly to thesubstrate or may be attached to the substrate 120 via one or moreintermediate material layers.

“Cavity” has its conventional meaning of “an empty space within a solidbody.” The cavity 140 may be a hole completely through the substrate 120(as shown in Section A-A and Section B-B) or a recess in the substrate120. The cavity 140 may be formed, for example, by selective etching ofthe substrate 120 before or after the piezoelectric plate 110 and thesubstrate 120 are attached.

The conductor pattern of the XBAR 100 includes an interdigitaltransducer (IDT) 130. The IDT 130 includes a first plurality of parallelfingers, such as finger 136, extending from a first busbar 132 and asecond plurality of fingers extending from a second busbar 134. Thefirst and second pluralities of parallel fingers are interleaved. Theinterleaved fingers overlap for a distance AP, commonly referred to asthe “aperture” of the IDT. The center-to-center distance L between theoutermost fingers of the IDT 130 is the “length” of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR100. A radio frequency or microwave signal applied between the twobusbars 132, 134 of the IDT 130 excites a primary acoustic mode withinthe piezoelectric plate 110. As will be discussed in further detail, theprimary acoustic mode is a bulk shear mode where acoustic energypropagates back and forth along a direction substantially orthogonal tothe surface of the piezoelectric plate 110, which is also normal, ortransverse, to the primary direction of the electric field createdbetween the IDT fingers. Thus, the XBAR is considered atransversely-excited film bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that atleast the fingers of the IDT 130 are disposed on the portion 115 of thepiezoelectric plate that spans, or is suspended over, the cavity 140. Asshown in FIG. 1, the cavity 140 has a rectangular shape with an extentgreater than the aperture AP and length L of the IDT 130. A cavity of anXBAR may have a different shape, such as a regular or irregular polygon.The cavity of an XBAR may have more or fewer than four sides, which maybe straight or curved.

For ease of presentation in FIG. 1, the geometric pitch and width of theIDT fingers is greatly exaggerated with respect to the length (dimensionL) and aperture (dimension AP) of the XBAR. A typical XBAR has more thanten parallel fingers in the IDT 110. An XBAR may have hundreds, possiblythousands, of parallel fingers in the IDT 110. Similarly, the thicknessof the fingers in the cross-sectional views is greatly exaggerated.

FIG. 2 shows a detailed schematic cross-sectional view of the XBAR 100.The piezoelectric plate 110 is a single-crystal layer of Lithium Niobatepiezoelectrical material having parallel front and back surfaces 112,114 and a thickness ts. ts may be, for example, 100 nm to 1500 nm. Whenused in filters for LTE™ bands from 3.4 GHZ to 6 GHz (e.g. LTE™ bands42, 43, 46), the thickness ts may be, for example, 300 nm to 700 nm.

The IDT fingers, such as IDT finger 236, may be disposed on the frontsurface 112 of the piezoelectric plate 110. Alternatively, IDT fingers,such as IDT finger 238, may be disposed in grooves formed in the frontsurface 112. The IDT fingers 236, 238 may be aluminum, substantiallyaluminum alloys, copper, substantially copper alloys, beryllium, gold,tungsten, molybdenum or some other conductive material. Thin (relativeto the total thickness of the conductors) layers of other metals, suchas chromium or titanium, may be formed under and/or over the fingers toimprove adhesion between the fingers and the piezoelectric plate 110and/or to passivate or encapsulate the fingers. The busbars (132, 134 inFIG. 1) of the IDT may be made of the same or different materials as thefingers.

Dimension p is the center-to-center spacing or “pitch” of the IDTfingers, which may be referred to as the pitch of the IDT and/or thepitch of the XBAR. Dimension w is the width or “mark” of the IDTfingers. The IDT of an XBAR differs substantially from the IDTs used insurface acoustic wave (SAW) resonators. In a SAW resonator, the pitch ofthe IDT is one-half of the wavelength of the horizontally-propagatingsurface acoustic wave at the resonance frequency. Additionally, themark-to-pitch ratio of a SAW resonator IDT is typically close to 0.5(i.e. the mark or finger width is about one-fourth of the acousticwavelength at resonance). In an XBAR, the pitch p of the IDT istypically 2 to 20 times the width w of the fingers. In addition, thepitch p of the IDT is typically 2 to 20 times the thickness ts of thepiezoelectric plate 112. The width of the IDT fingers in an XBAR is notconstrained to one-fourth of the acoustic wavelength at resonance. Forexample, the width of XBAR IDT fingers may be 500 nm or greater, suchthat the IDT can be fabricated using optical lithography.

The thickness tm of the IDT fingers may be from 100 nm to about equal tothe width w. The thickness of the busbars (132, 134 in FIG. 1) of theIDT may be the same as, or greater than, the thickness tm of the IDTfingers. The depth tg of the grooves formed in the front surface may beless than (as shown in FIG. 2), equal to, or greater than tm, but notgreater than ts

A front-side dielectric layer 214 may optionally be formed on the frontsurface 112 of the piezoelectric plate 110. The “front side” of the XBARis, by definition, the surface facing away from the substrate. Thefront-side dielectric layer 214 has a thickness tfd. The front-sidedielectric layer 214 is formed between the IDT fingers 236, 238.Although not shown in FIG. 2, the front side dielectric layer 214 mayalso be deposited over the IDT fingers 238. The front-side dielectriclayer 214 may be a non-piezoelectric dielectric material, such assilicon dioxide or silicon nitride. tfd may be, for example, 0 to 500nm. tfd is typically less than the thickness ts of the piezoelectricplate. The front-side dielectric layer 214 may be formed of multiplelayers of two or more materials. The front-side dielectric layer may bedeposited, for example by evaporation, sputtering, chemical vapordeposition, or some other technique.

FIG. 3 shows a detailed schematic cross-sectional view of a solidlymounted XBAR (SM XBAR) 300. SM XBARs were first described in applicationSer. No. 16/381,141. The SM XBAR 300 includes a piezoelectric plate 110,an IDT (of which only fingers 336, 338 are visible) and an optionalfront-side dielectric layer 214 as previously described. Thepiezoelectric layer 110 has parallel front and back surfaces 112, 114.Dimension ts is the thickness of the piezoelectric plate 110. The widthof the IDT fingers 336, 338 is dimension w, thickness of the IDT fingersis dimension tm, and the IDT pitch is dimension p. The thickness of thefront-side dielectric layer 214 is dimension tfd.

In contrast to the XBAR devices shown in FIG. 1 and FIG. 2, the IDT ofan SM XBAR is not formed on a diaphragm spanning a cavity in thesubstrate 120. Instead, an acoustic Bragg reflector 340 is sandwichedbetween a surface 222 of the substrate 220 and the back surface 114 ofthe piezoelectric plate 110. The term “sandwiched” means the acousticBragg reflector 340 is both disposed between and mechanically attachedto a surface 222 of the substrate 220 and the back surface 114 of thepiezoelectric plate 110. In some circumstances, thin layers ofadditional materials may be disposed between the acoustic Braggreflector 340 and the surface 222 of the substrate 220 and/or betweenthe Bragg reflector 340 and the back surface 114 of the piezoelectricplate 110. Such additional material layers may be present, for example,to facilitate bonding the piezoelectric plate 110, the acoustic Braggreflector 340, and the substrate 220.

The acoustic Bragg reflector 340 includes multiple dielectric layersthat alternate between materials having high acoustic impedance andmaterials have low acoustic impedance. The acoustic impedance of amaterial is the product of the material's shear wave velocity anddensity. “High” and “low” are relative terms. For each layer, thestandard for comparison is the adjacent layers. Each “high” acousticimpedance layer has an acoustic impedance higher than that of both theadjacent low acoustic impedance layers. Each “low” acoustic impedancelayer has an acoustic impedance lower than that of both the adjacenthigh acoustic impedance layers. As will be discussed subsequently, theprimary acoustic mode in the piezoelectric plate of an XBAR is a shearbulk wave. Each layer of the acoustic Bragg reflector 340 has athickness equal to, or about, one-fourth of the wavelength in the layerof a shear bulk wave having the same polarization as the primaryacoustic mode at or near a resonance frequency of the SM XBAR 300.Dielectric materials having comparatively low acoustic impedance includesilicon dioxide, carbon-containing silicon oxide, and certain plasticssuch as cross-linked polyphenylene polymers. Materials havingcomparatively high acoustic impedance include hafnium oxide, siliconnitride, aluminum nitride, silicon carbide. All of the high acousticimpedance layers of the acoustic Bragg reflector 340 are not necessarilythe same material, and all of the low acoustic impedance layers are notnecessarily the same material. In the example of FIG. 3, the acousticBragg reflector 340 has a total of six layers. An acoustic Braggreflector may have more than, or less than, six layers.

The IDT fingers, such as IDT finger 336, may be disposed on the frontsurface 112 of the piezoelectric plate 110. Alternatively, IDT fingers,such as IDT fingers 238 in FIGS. 2 and 338 in FIG. 3, may be disposed ingroves formed in the front surface 112. The grooves may extend partiallythrough the piezoelectric plate, as shown in FIG. 2. Alternatively, thegrooves may extend completely through the piezoelectric plate as shownin FIG. 3.

FIG. 4 is a graphical illustration of the primary acoustic mode in anXBAR. FIG. 4 shows a small portion of an XBAR 400 including apiezoelectric plate 410 and three interleaved IDT fingers 430. An RFvoltage is applied to the interleaved fingers 430. This voltage createsa time-varying electric field between the fingers. The direction of theelectric field is predominantly lateral, or parallel to the surface ofthe piezoelectric plate 410, as indicated by the arrows labeled“electric field”. Due to the high dielectric constant of thepiezoelectric plate, the electric energy is highly concentrated in theplate relative to the air. The lateral electric field introduces sheardeformation, and thus strongly excites a shear-mode acoustic mode, inthe piezoelectric plate 410. In this context, “shear deformation” isdefined as deformation in which atomic displacements are horizontal butvary in a vertical direction. A “shear acoustic mode” is defined as anacoustic vibration mode in a medium that results in shear deformation ofthe medium. The shear deformations in the XBAR 400 are represented bythe curves 460, with the adjacent small arrows providing a schematicindication of the direction and magnitude of physical motion of thepiezoelectric media. The degree of physical motion, as well as thethickness of the piezoelectric plate 410, have been greatly exaggeratedfor ease of visualization. While the atomic motions are predominantlylateral (i.e. horizontal as shown in FIG. 4), the direction of acousticenergy flow of the excited primary shear acoustic mode is substantiallyorthogonal to the surface of the piezoelectric plate, as indicated bythe arrow 465.

In an SM XBAR, as shown in FIG. 3, the motion distribution in thepiezoelectric plate is similar. However, the thickness of the plate isnot necessarily close to one-half of the wavelength of the primaryacoustic mode, and some part of acoustic energy is localized in theBragg stack, in shear vibrations with amplitude exponentially decayingin the depth of the stack.

Considering FIG. 4, there is essentially no horizontal electric fieldimmediately under the IDT fingers 430, and thus acoustic modes are onlyminimally excited in the regions 470 under the fingers. There may beevanescent acoustic motions in these regions. Since acoustic vibrationsare not excited under the IDT fingers 430, the acoustic energy coupledto the metal IDT fingers 430 is low (for example compared to the fingersof an IDT in a SAW resonator), which reduces viscous losses in the IDTfingers.

An acoustic resonator based on shear acoustic wave resonances canachieve better performance than current state-of-the artfilm-bulk-acoustic-resonators (FBAR) and solidly-mounted-resonatorbulk-acoustic-wave (SMR BAW) devices where the electric field is appliedin the thickness direction. In such devices, the acoustic mode iscompressive with atomic motions and the direction of acoustic energyflow in the thickness direction. This compression/extension of theelastic media is responsible for additional adiabatic loss mechanismabsent for pure shear waves. In addition, the strongest coupling inlithium niobate and lithium tantalate corresponds to the sheardeformations. Thus, the piezoelectric coupling for shear wave XBARresonances can be high (>20%) compared to other acoustic resonators.High piezoelectric coupling enables the design and implementation ofmicrowave and millimeter-wave filters with appreciable bandwidth.

FIG. 5 is a graphical illustration of Euler angles. Euler angles are asystem, introduced by Swiss mathematician Leonhard Euler, to define theorientation of a body with respect to a fixed coordinate system. Theorientation is defined by three successive rotations about angles α, β,and γ.

As applied to acoustic wave devices, XYZ is a three-dimensionalcoordinate system aligned with the crystalline axes of the piezoelectricmaterial. xyz is a three-dimensional coordinate system aligned with theacoustic wave device, where the z axis is normal to the surface of thepiezoelectric material. xy is the plane of the surface of thepiezoelectric material. In acoustic wave devices excited by an IDT, thex axis is usually defined as normal to the fingers of an IDT and the yaxis is usually defined as parallel to the fingers of the IDT. In thiscase, the electric field is primarily along the x axis. In most devicesexcited by an IDT, there will be components of the electric field alongthe y axis (e.g. at the ends of the IDT fingers) and the z axis (e.g.under the fingers of the IDT). The vector N is the intersection of thexy and XY planes. The vector N is also the common perpendicular to the zand Z axis.

The electric field applied to a piezoelectric material can be describedby a three-element vector, where the elements of the vector are theelectric field components along the X, Y, and Z crystalline axes. Thestress introduced in a piezoelectric material by an applied electricfield can be determined by multiplying the electric field vector by amatrix of piezoelectric coefficients. When the xyz axes of a physicalacoustic wave device are rotated with respect to the XYZ axes of thepiezoelectric material, the piezoelectric coefficients depend on therotation angles.

Although application Ser. No. 16/230,443 and application Ser. No.16/381,141 are not limited to a specific type or orientation of apiezoelectric material, all of the examples in those applications useLithium Tantalate or Lithium Niobate piezoelectric plates with the Zcrystalline axis normal to the plate surface and the Y crystalline axisorthogonal to the IDT fingers. Such piezoelectric plates have Eulerangles of [0°, 0°, 90° ].

FIG. 6 is a graph 600 of piezoelectric coefficient e24 relating electricfield along the Y axis to shear stress or torque about the ε_(yz) axisfor lithium niobate plates with Euler angles of [0°, β, 0° ]. The solidline 610 is a plot of this piezoelectric coefficient as a function ofthe Euler angle β. Shear stress or torque ε_(yz) excites the primaryshear acoustic mode shown in FIG. 4. Inspection of FIG. 6 shows that thepiezoelectric stress coefficient is high over a range −15°≤β<0°. Thepiezoelectric coefficient e24 reaches a highest value of about 3.9 atβ=−7.5°.

Application Ser. No. 16/518,594 describes XBAR devices on piezoelectricplates with Euler angles [0°, β, 90° ], where −15°≤β<0°. As an inferredfrom FIG. 6, such XBAR devices have higher piezoelectric coupling thandevices on piezoelectric plates with Euler angles of [0°, 0°, 90° ]. Forexample, the electromechanical coupling coefficient is 0.263 for β equalto −7.5°, as compared to a value of about 0.243 for β=0°.

This patent is directed to XBAR devices on lithium niobate plates havingEuler angles [0°, β, 0° ]. For historical reasons, this plateconfiguration is commonly referred to as “Y-cut”, where the “cut angle”is the angle between the y axis and the normal to the plate. The “cutangle” is equal to β+90°. For example, a plate with Euler angles [0°,30°, 0° ] is commonly referred to as “120° rotated Y-cut”.

FIG. 7 is a graph 700 of two piezoelectric coefficients e15 and e16 forlithium niobate plates having Euler angles [0°, β, 0° ]. The solid line710 is a plot of piezoelectric coefficient e15 relating electric fieldalong the x axis to shear stress or torque ε_(XZ) axis as a function ofβ. This shear stress excites the shear primary acoustic mode shown inFIG. 4. The dashed line 720 is a plot of piezoelectric coefficient e16relating electric field along the x axis to shear stress or torqueε_(xy) as a function of β. This shear stress excites horizontal shearmodes (e.g. the SHO plate mode) with atomic displacements normal to theplane of FIG. 4, which are undesired parasitic modes in an XBAR. Notethat these two curves are identical and shifted by 90°, (as y-axisshifted from x-axis).

Inspection of FIG. 7 shows that the first piezoelectric stresscoefficient is highest for Euler angle β about 30°. The firstpiezoelectric stress coefficient is higher than about 3.8 (the highestpiezoelectric stress coefficient for an unrotated Z-cut lithium niobate)for 0°≤β≤60°. The second piezoelectric stress coefficient is zero forEuler angle β about 30°, where the first piezoelectric stresscoefficient is maximum. In this context “about 30°” means “within areasonable manufacturing tolerance of 30°”. The second piezoelectricstress coefficient is less than about 10% of the first piezoelectricstress coefficient for 26°≤β≤34°.

FIG. 8 is a chart 800 showing the normalized magnitude of the admittance(on a logarithmic scale) as a function of frequency for two XBAR devicessimulated using finite element method (FEM) simulation techniques. Thedashed line 820 is a plot of the admittance on an XBAR on a Z-cutlithium niobate plate. In this case the Z crystalline axis is orthogonalto the surfaces of the plate, the electric field is applied along the Ycrystalline axis, and the Euler angles of the piezoelectric plate are 0,0, 90°. The solid line 810 is a plot of the admittance of an XBAR on a120° Y-cut lithium niobate plate. In this case, the electric field isapplied along the crystalline X axis, which lies in the plane of thesurfaces of the lithium niobate plate. The YZ plane is normal to thesurfaces of the plate. The Z crystalline axis is inclined by 30° withrespect to orthogonal to the surfaces of the plate and the Euler anglesof the piezoelectric plate are 0°, 30°, 0°. In both cases, the platethickness is 400 nm, and the IDT fingers are aluminum 100 nm thick. Thesubstrate supporting the piezoelectric plate is silicon with a cavityformed under the IDT fingers.

The difference between anti-resonance and resonance frequencies of theresonator on the rotated Y-cut plate (solid line 810) is about 200 MHzgreater than the difference between anti-resonance and resonancefrequencies of the resonator on the Z-cut plate (dashed line 820). Theelectromechanical coupling of the XBAR on the rotated Y-cut plate isabout 0.32; the electromechanical coupling of the XBAR on the Z-cutplate is about 0.24.

FIG. 9 is a chart 900 comparing the anti-resonance and resonancefrequencies of XBAR resonators on lithium niobate plates having Eulerangles 0°, β, 0°. The lithium niobate plate thickness is 500 nm. Thesolid line 910 is a plot of anti-resonance frequency as a function of β.The dashed lint 920 is a plot of resonance frequency as a function of β.Consideration of the plots shows that a both resonance andanti-resonance frequencies have a slight dependence on β and thedifference between the resonance and anti-resonance frequencies isreasonably constant over the range 27°≤β≤40°.

FIG. 10 is a schematic circuit diagram for an exemplary high frequencyband-pass filter 1000 using XBARs. The filter 1000 has a conventionalladder filter architecture including four series resonators 1010A,1010B, 1010C, 1010D and three shunt resonators 1020A, 1020B, 1020C. Thefour series resonators 1010A, 1010B, 1010C and 1010D are connected inseries between a first port and a second port. In FIG. 10, the first andsecond ports are labeled “In” and “Out”, respectively. However, thefilter 1000 is symmetrical and either port and serve as the input oroutput of the filter. The three shunt resonators 1020A, 1020B, 1020C areconnected from nodes between the series resonators to ground. All theshunt resonators and series resonators in this example are XBARs.

The four series resonators 1010A, B, C, D and the three shunt resonators1020A, B, C of the filter 1000 may be formed on a single plate 1030 ofpiezoelectric material bonded to a silicon substrate (not visible). Eachresonator includes a respective IDT (not shown) with least the fingersof each IDT are disposed over a cavity in the substrate. In FIG. 10, thecavities are illustrated schematically as the dashed rectangles (such asthe rectangle 1035). In this example, each IDT is disposed over arespective cavity. In this and similar contexts, the term “respective”means “relating things each to each”, which is to say with a one-to-onecorrespondence. In other filters, the IDTs of two or more resonators maybe disposed over a common cavity.

A similar filter could be designed using SM XBARs. When the resonatorsare SM XBARs, the IDTs are disposed over an acoustic Bragg reflector asshown in FIG. 3.

FIG. 11 is a chart 1100 showing simulated performance of an embodimentof the band-pass filter 1100. The XBARs are formed on a 128° Y-cutlithium niobate plate Euler angles 0°, 38°, 0°). The thickness is of thelithium niobate plate is 500 nm. The substrate is silicon, the IDTconductors are aluminum, the front-side dielectric, where present, isSiO2. The thickness tm of the IDT fingers is 650 nm, such thattm/ts=1.30. The other physical parameters of the resonators are providedin the following table (all dimensions are in microns; p=IDT pitch,w=IDT finger width, and tfd=front-side dielectric thickness):

Series Resonators Shunt Resonators Parameter X1 X3 X5 X7 X2 X4 X6 P 3.433.45 3.47 3.02 5.44 5.04 5.18 w 1.14 1.16 1.18 1.14 1.10 1.14 1.12 tfd 00 0 0 0.17 0.17 0.17

In FIG. 11, the solid line 1110 is a plot of S(1,2), which is theinput-output transfer function of the filter, as a function offrequency. The dashed line 1120 is a plot of S(1,1), which is thereflection at the input port, as a function of frequency. The dash-dotvertical lines delimit band N77 from 3.3 to 4.2 GHz. Both plots 1110,1120 are based on circuit simulation using finite element methods tomodel the resonators.

Description of Methods

FIG. 12 is a simplified flow chart showing a process 1200 for making anXBAR or a filter incorporating XBARs. The process 1200 starts at 1205with a substrate and a plate of piezoelectric material and ends at 1295with a completed XBAR or filter. The flow chart of FIG. 12 includes onlymajor process steps. Various conventional process steps (e.g. surfacepreparation, cleaning, inspection, baking, annealing, monitoring,testing, etc.) may be performed before, between, after, and during thesteps shown in FIG. 12.

The flow chart of FIG. 12 captures three variations of the process 1200for making an XBAR which differ in when and how cavities are formed inthe substrate. The cavities may be formed at steps 1210A, 1210B, or1210C. Only one of these steps is performed in each of the threevariations of the process 1200.

The piezoelectric plate may be, for example, rotated Y-cut lithiumniobate. The Euler angles of the piezoelectric plate are [0°, β, 0° ],where β is in the range from 0° to 60°. Preferably, β may be in therange from 26° to 34° to minimize coupling into hear horizontal acousticmodes. β may be about 30° The substrate may preferably be silicon. Thesubstrate may be some other material that allows formation of deepcavities by etching or other processing.

In one variation of the process 1200, one or more cavities are formed inthe substrate at 1210A, before the piezoelectric plate is bonded to thesubstrate at 1220. A separate cavity may be formed for each resonator ina filter device. The one or more cavities may be formed usingconventional photolithographic and etching techniques. Typically, thecavities formed at 1210A will not penetrate through the substrate.

At 1220, the piezoelectric plate is bonded to the substrate. Thepiezoelectric plate and the substrate may be bonded by a wafer bondingprocess. Typically, the mating surfaces of the substrate and thepiezoelectric plate are highly polished. One or more layers ofintermediate materials, such as an oxide or metal, may be formed ordeposited on the mating surface of one or both of the piezoelectricplate and the substrate. One or both mating surfaces may be activatedusing, for example, a plasma process. The mating surfaces may then bepressed together with considerable force to establish molecular bondsbetween the piezoelectric plate and the substrate or intermediatematerial layers.

A conductor pattern, including IDTs of each XBAR, is formed at 1230 bydepositing and patterning one or more conductor layer on the front sideof the piezoelectric plate. The conductor layer may be, for example,aluminum, an aluminum alloy, copper, a copper alloy, or some otherconductive metal. Optionally, one or more layers of other materials maybe disposed below (i.e. between the conductor layer and thepiezoelectric plate) and/or on top of the conductor layer. For example,a thin film of titanium, chrome, or other metal may be used to improvethe adhesion between the conductor layer and the piezoelectric plate. Aconduction enhancement layer of gold, aluminum, copper or other higherconductivity metal may be formed over portions of the conductor pattern(for example the IDT bus bars and interconnections between the IDTs).

The conductor pattern may be formed at 1230 by depositing the conductorlayer and, optionally, one or more other metal layers in sequence overthe surface of the piezoelectric plate. The excess metal may then beremoved by etching through patterned photoresist. The conductor layercan be etched, for example, by plasma etching, reactive ion etching, wetchemical etching, and other etching techniques.

Alternatively, the conductor pattern may be formed at 1230 using alift-off process. Photoresist may be deposited over the piezoelectricplate and patterned to define the conductor pattern. The conductor layerand, optionally, one or more other layers may be deposited in sequenceover the surface of the piezoelectric plate. The photoresist may then beremoved, which removes the excess material, leaving the conductorpattern.

At 1240, a front-side dielectric layer may be formed by depositing oneor more layers of dielectric material on the front side of thepiezoelectric plate. The one or more dielectric layers may be depositedusing a conventional deposition technique such as sputtering,evaporation, or chemical vapor deposition. The one or more dielectriclayers may be deposited over the entire surface of the piezoelectricplate, including on top of the conductor pattern. Alternatively, one ormore lithography processes (using photomasks) may be used to limit thedeposition of the dielectric layers to selected areas of thepiezoelectric plate, such as only between the interleaved fingers of theIDTs. Masks may also be used to allow deposition of differentthicknesses of dielectric materials on different portions of thepiezoelectric plate.

In a second variation of the process 1200, one or more cavities areformed in the back side of the substrate at 1210B. A separate cavity maybe formed for each resonator in a filter device. The one or morecavities may be formed using an anisotropic or orientation-dependent dryor wet etch to open holes through the back side of the substrate to thepiezoelectric plate. In this case, the resulting resonator devices willhave a cross-section as shown in FIG. 1.

In the second variation of the process 1200, a back-side dielectriclayer may be formed at 1250. In the case where the cavities are formedat 1210B as holes through the substrate, the back-side dielectric layermay be deposited through the cavities using a conventional depositiontechnique such as sputtering, evaporation, or chemical vapor deposition.

In a third variation of the process 1200, one or more cavities in theform of recesses in the substrate may be formed at 1210C by etching thesubstrate using an etchant introduced through openings in thepiezoelectric plate. A separate cavity may be formed for each resonatorin a filter device.

In all variations of the process 1200, the filter device is completed at1260. Actions that may occur at 1260 include depositing anencapsulation/passivation layer such as SiO₂ or Si₃O₄ over all or aportion of the device; forming bonding pads or solder bumps or othermeans for making connection between the device and external circuitry;excising individual devices from a wafer containing multiple devices;other packaging steps; and testing. Another action that may occur at1260 is to tune the resonant frequencies of the resonators within thedevice by adding or removing metal or dielectric material from the frontside of the device. After the filter device is completed, the processends at 1295.

FIG. 13 is a simplified flow chart of a method 1300 for making a SM XBARor a filter incorporating SM XBARs. The method 1300 starts at 1310 witha thin piezoelectric plate disposed on a sacrificial substrate 1302 anda device substrate 1304. The method 1300 ends at 1395 with a completedSM XBAR or filter. The flow chart of FIG. 13 includes only major processsteps. Various conventional process steps (e.g. surface preparation,cleaning, inspection, baking, annealing, monitoring, testing, etc.) maybe performed before, between, after, and during the steps shown in FIG.13.

The piezoelectric plate 1302 may be, for example, rotated Z-cut lithiumniobate. The Euler angles of the piezoelectric plate are [0°, β, 0° ],where β is in the range from 0° to 60°. Preferably, β may be in therange from 26° to 34° to minimize coupling to shear horizontal modes. βmay be about 30°

At 1320, an acoustic Bragg reflector is formed by depositing alternatingdielectric layers of high acoustic impedance and low acoustic impedancematerials. Each of the layers has a thickness equal to or aboutone-fourth of the acoustic wavelength. Dielectric materials havingcomparatively low acoustic impedance include silicon dioxide, siliconoxycarbide, and certain plastics such as cross-linked polyphenylenepolymers. Dielectric materials having comparatively high acousticimpedance include silicon nitride and aluminum nitride. All of the highacoustic impedance layers are not necessarily the same material, and allof the low acoustic impedance layers are not necessarily the samematerial. The total number of layers in the acoustic Bragg reflector maybe from about five to more than twenty.

At 1320, all of the layers of the acoustic Bragg reflector may bedeposited on either the surface of the piezoelectric plate on thesacrificial substrate 1302 or a surface of the device substrate 1304.Alternatively, some of the layers of the acoustic Bragg reflector may bedeposited on the surface of the piezoelectric plate on the sacrificialsubstrate 1302 and the remaining layers of the acoustic Bragg reflectormay be deposited on a surface of the device substrate 1304.

At 1330, the piezoelectric plate on the sacrificial substrate 1302 andthe device substrate 1304 may be bonded such that the layers of theacoustic Bragg reflector are sandwiched between the piezoelectric plateand the device substrate. The piezoelectric plate on the sacrificialsubstrate 1302 and the device substrate 1304 may be bonded using a waferbonding process such as direct bonding, surface-activated orplasma-activated bonding, electrostatic bonding, or some other bondingtechnique. Note that, when one or more layers of the acoustic Braggreflector are deposited on both the piezoelectric plate and the devicesubstrate, the bonding will occur between or within layers of theacoustic Bragg reflector.

After the piezoelectric plate on the sacrificial substrate 1302 and thedevice substrate 1304 may be bonded, the sacrificial substrate, and anyintervening layers, are removed at 1340 to expose the surface of thepiezoelectric plate (the surface that previously faced the sacrificialsubstrate). The sacrificial substrate may be removed, for example, bymaterial-dependent wet or dry etching or some other process.

A conductor pattern, including IDTs of each SM XBAR, is formed at 1350by depositing and patterning one or more conductor layer on the surfaceof the piezoelectric plate that was exposed when the sacrificialsubstrate was removed at 1340. The conductor pattern may be, forexample, aluminum, an aluminum alloy, copper, a copper alloy, or someother conductive metal. Optionally, one or more layers of othermaterials may be disposed below (i.e. between the conductor layer andthe piezoelectric plate) and/or on top of the conductor layer. Forexample, a thin film of titanium, chrome, or other metal may be used toimprove the adhesion between the conductor layer and the piezoelectricplate. A conduction enhancement layer of gold, aluminum, copper or otherhigher conductivity metal may be formed over portions of the conductorpattern (for example the IDT bus bars and interconnections between theIDTs).

The conductor pattern may be formed at 1350 by depositing the conductorlayer and, optionally, one or more other metal layers in sequence overthe surface of the piezoelectric plate. The excess metal may then beremoved by etching through patterned photoresist. The conductor layercan be etched, for example, by plasma etching, reactive ion etching, wetchemical etching, and other etching techniques.

Alternatively, the conductor pattern may be formed at 1350 using alift-off process. Photoresist may be deposited over the piezoelectricplate and patterned to define the conductor pattern. The conductor layerand, optionally, one or more other layers may be deposited in sequenceover the surface of the piezoelectric plate. The photoresist may then beremoved, which removes the excess material, leaving the conductorpattern.

At 1360, one or more optional front-side dielectric layers may be formedby depositing one or more layers of dielectric material on the frontside of the piezoelectric plate. The one or more dielectric layers maybe deposited using a conventional deposition technique such assputtering, evaporation, or chemical vapor deposition. The one or moredielectric layers may be deposited over the entire surface of thepiezoelectric plate, including on top of the conductor pattern.Alternatively, one or more lithography processes (using photomasks) maybe used to limit the deposition of the dielectric layers to selectedareas of the piezoelectric plate, such as only between the interleavedfingers of the IDTs. Masks may also be used to allow deposition ofdifferent thicknesses of dielectric materials on different portions ofthe piezoelectric plate. For example, a first dielectric layer having afirst thickness t1 may be deposited over the IDTs of one or more shuntresonators. A second dielectric layer having a second thickness t2,where t2 is equal to or greater than zero and less than t1, may bedeposited over the IDTs of series resonators.

After the conductor pattern and optional front-side dielectric layer areformed at 1350 and 1360, the filter device may be completed at 1370.Actions that may occur at 1370 including depositing and patterningadditional metal layers to form conductors other than the IDT conductorpattern; depositing an encapsulation/passivation layer such as SiO₂ orSi₃O₄ over all or a portion of the device; forming bonding pads orsolder bumps or other means for making connection between the device andexternal circuitry; excising individual devices from a wafer containingmultiple devices; other packaging steps; and testing. Another actionthat may occur at 1370 is to tune the resonant frequencies of theresonators within the device by adding or removing metal or dielectricmaterial from the front side of the device. After the filter device iscompleted, the process ends at 1395.

A variation of the process 1300 starts with a single-crystalpiezoelectric wafer at 1302 instead of a thin piezoelectric plate on asacrificial substrate of a different material. Ions are implanted to acontrolled depth beneath a surface of the piezoelectric wafer (not shownin FIG. 13). The portion of the wafer from the surface to the depth ofthe ion implantation is (or will become) the thin piezoelectric plateand the balance of the wafer is the sacrificial substrate. The acousticBragg reflector is formed at 1320 as previously described and thepiezoelectric wafer and device substrate are bonded at 1330 such thatthe acoustic Bragg reflector is disposed between the ion-implantedsurface of the piezoelectric wafer 1302 and the device substrate 1304.At 1340, the piezoelectric wafer may be split at the plane of theimplanted ions (for example, using thermal shock), leaving a thin plateof piezoelectric material exposed and bonded to the acoustic Braggreflector. The thickness of the thin plate piezoelectric material isdetermined by the energy (and thus depth) of the implanted ions. Theprocess of ion implantation and subsequent separation of a thin plate iscommonly referred to as “ion slicing”.

CLOSING COMMENTS

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. An acoustic resonator device comprising: a substratehaving a surface; a lithium niobate plate having front and backsurfaces, the back surface attached to the surface of the substrateexcept for a portion of the lithium niobate plate forming a diaphragmthat spans a cavity in the substrate; and an interdigital transducer(IDT) formed on the front surface of the lithium niobate plate such thatinterleaved fingers of the IDT are disposed on the diaphragm, whereinthe lithium niobate plate and the IDT are configured such that a radiofrequency signal applied to the IDT excites a primary shear acousticmode in the diaphragm, and Euler angles of the lithium niobate plate are[0°, β, 0° ], where β is greater than or equal to 0° and less than orequal to 60°.
 2. The device of claim 1, wherein β is greater than orequal to 26° and less than or equal to 34°.
 3. The device of claim 1,wherein β is about 30°.
 4. The device of claim 1, wherein a direction ofacoustic energy flow of the primary acoustic mode is substantiallyorthogonal to the front and back surfaces of the diaphragm.
 5. Thedevice of claim 1, wherein a thickness between the front and backsurfaces of the lithium niobate plate is greater than or equal to 200 nmand less than or equal to 1000 nm.
 6. The device of claim 5, wherein apitch of the fingers of the IDT is greater than or equal to 2 times thethickness of the lithium niobate plate and less than or equal to 25times the thickness of the lithium niobate.
 7. The device of claim 6,wherein the fingers of the IDT have a width, and the pitch is greaterthan or equal to 2 times the width and less than or equal to 25 timesthe width.
 8. The device of claim 1, further comprising: a front-sidedielectric layer formed on the front surface of the lithium niobateplate between the fingers of the IDT.
 9. A filter device, comprising: asubstrate; a lithium niobate plate having front and back surfaces, theback surface attached to the surface of the substrate, portions of thelithium niobate plate forming one or more diaphragms spanning respectivecavities in the substrate; and a conductor pattern formed on the frontsurface, the conductor pattern including a plurality of interdigitaltransducers (IDTs) of a respective plurality of acoustic resonators,interleaved fingers of each of the plurality of IDTs disposed on the oneor more diaphragms, wherein the lithium niobate plate and all of theIDTs are configured such that respective radio frequency signals appliedto the IDTs excite respective primary shear acoustic modes in therespective diaphragms, and the Euler angles of the lithium niobate plateare [0°, β, 0° ], where β is greater than or equal to 0° and less thanor equal to 60°.
 10. The filter device of claim 9, wherein β is greaterthan or equal to 26° and less than or equal to 34°.
 11. The filterdevice of claim 9, wherein β is about 30°.
 12. The filter device ofclaim 9, wherein a direction of acoustic energy flow of all of theprimary acoustic mode is substantially orthogonal to the front and backsurfaces of the diaphragm.
 13. The filter device of claim 9, wherein athickness between the front and back surfaces of the lithium niobateplate is greater than or equal to 200 nm and less than or equal to 1000nm.
 14. The filter device of claim 13, wherein each of the plurality ofIDTs has a respective pitch greater than or equal to 2 times thethickness of the lithium niobate plate and less than or equal to 25times the thickness of the lithium niobate.
 15. The filter device ofclaim 14, wherein the fingers of each of the plurality of IDTs have arespective width, and for each of the plurality of IDTs, the respectivepitch is greater than or equal to 2 times the respective width and lessthan or equal to 25 times the respective width.
 16. The filter device ofclaim 9, wherein each of the plurality of IDTs is disposed on arespective diaphragm spanning a respective cavity.
 17. The filter deviceof claim 9, wherein the plurality of acoustic resonators includes ashunt resonator and a series resonator, and a thickness of a firstdielectric layer formed over the shunt resonator is greater than athickness of a second dielectric layer formed over the series resonator.18. A method of fabricating an acoustic resonator device, comprising:bonding a lithium niobate plate to a substrate such that a portion ofthe lithium niobate plate forms a diaphragm spanning a cavity in thesubstrate; and forming an interdigital transducer (IDT) on the frontsurface of the lithium niobate plate such that interleaved fingers ofthe IDT are disposed on the diaphragm, wherein the lithium niobate plateand the IDT are configured such that a radio frequency signal applied tothe IDT excites a primary shear acoustic mode in the diaphragm, and theEuler angles of the lithium niobate plate are [0°, β, 0° ], where β isgreater than or equal to 0° and less than 60°.
 19. The method of claim18, wherein β is greater than or equal to 26° and less than or equal to34°.
 20. The method of claim 18, wherein β is about 30°.
 21. The methodof claim 18, wherein a direction of acoustic energy flow of the primaryacoustic mode is substantially orthogonal to the front and back surfacesof the diaphragm.
 22. The method of claim 18, wherein a thicknessbetween the front and back surfaces of the lithium niobate plate isgreater than or equal to 200 nm and less than or equal to 1000 nm. 23.The method of claim 22, wherein a pitch of the fingers of the IDT isgreater than or equal to 2 times the thickness of the lithium niobateplate and less than or equal to 25 times the thickness of the lithiumniobate.
 24. The method of claim 23, wherein the fingers of the IDT havea width, and the pitch is greater than or equal to 2 times the width andless than or equal to 25 times the width.
 25. The method of claim 18,further comprising: forming a front-side dielectric layer on the frontsurface of the lithium niobate plate between the fingers of the IDT.